Dispensers of the type described above are already on the market. In these dispensers, the piston that pushes the product to be dispensed in the direction of the dispensing opening is mounted on the end of a threaded rod, which cooperates with a fixed nut integrally joined to the reservoir containing the product. The threaded rod is displaceable in rotation by means of a mechanism that converts the action of the user on a pushbutton of the dispenser into a rotation of slight angular amplitude; the rotation of the threaded rod, which cooperates with the aforementioned fixed nut, is converted into translation of the rod with respect to the reservoir, the amplitude of the translation being increasingly small, the smaller the rotation of the rod, on the one hand, and the smaller the pitch of the thread of the rod, on the other. For each action upon the pushbutton of the dispenser, a translation of several tenths of a millimeter is obtained, which is entirely satisfactory for dispensing the product. A dispenser of this type that can be used for a liquid product is described in particular in French Patent 2 555 471.
However, the mechanism disposed between the pushbutton and the piston is very expensive, partly because of the number of parts comprising it and partly because of the complexity of molding of the parts, as well as the difficulty of assembling the mechanism. The function of such a mechanism is irreversible; when the piston arrives in the vicinity of the reservoir dispensing opening, it cannot be made to move backward in order to refill the reservoir; the only way to reverse it would be to screw the threaded rod backward in its nut, but it is understandable that the user would not think of that. As a result, the mechanism of such a dispenser must be considered a throwaway item, and its cost considerably increases the cost price of the dispenser filled with the product to be dispensed.
To overcome these disadvantages, it has been proposed, in French Patent Application 88-05026 filed on April 15, 1988, that the support of the mechanism enabling the incremental rotational displacement of the threaded rod, which is likewise the support or the nut, includes a means of integral attachment to the reservoir, making the reservoir detachable with respect to this support; that the nut, which cooperates with the threaded rod, is slit along at least one diametral plane over the entire height of its threaded bore, with the sectors of the nut then being kept together by a flexible zone that assures the fixation of the nut on the support; and that the reservoir includes a frustoconical bearing that when the detachable reservoir is attached to the support presses against a corresponding frustoconical bearing of the nut and/or support, in order to assure the squeezing of the sectors of the nut around the threaded rod driven by the mechanism. With such a structure, the intended object can be attained, that is, to re-use the same mechanism for dispensing the product from several reservoirs in succession. Nevertheless, the mechanism itself remains the same as the type described in French Patent 2 555 471, that is, a mechanism, that is intrinsically very expensive, for the reasons already given above.